The present invention relates to a system for carrying out communication between a plurality of electronic apparatus with the electronic apparatus being connected using an IEEE 1394 serial bus etc., and more particularly relates to technology for reducing the number of communication of control signals used in communicating while a certain electronic apparatus controls a plurality of other electronic apparatus.
A system has been considered where a plurality of electronic apparatus (hereinafter referred to as "apparatus") are connected together by a bus such as an IEEE 1394 serial bus (hereinafter referred to as a "1394 bus") which is capable of transmitting information signal packets and control signal packets with these packets mixed together and communication is then carried out between these apparatus.
In this system, time division multiplexing is performed on transmitted signals each prescribed communication cycle (for example, 125 msecs) as shown in FIG. 1. This signal transmission commences when an apparatus referred to as a "cycle master" transmits a cycle start packet indicating the start of a communications cycle onto the 1394 bus. The two types of communication occurring during one communication cycle are isochronous (hereinafter referred to as "Iso") communication where information signals such as digital video signals and digital audio signals etc. are transmitted in real time and asynchronous (hereinafter referred to as "Async") communication for transmitting control signals such as operation control commands for apparatus or control commands for connection between apparatus in irregular periods as necessary. Here, Iso packets are transmitted before Async packets. A plurality of Iso data can then be discriminated between by giving channel numbers 1, 2, 3, . . . n to the Iso packets. After the transmission of all of the Iso packets to be transmitted is complete, the period until the next cycle start packet is used for transmitting Async packets.
In Async communications, a control signal by which a certain apparatus requests something of another apparatus is referred to as a "command" and the side transmitting this command is referred to as a controller, with the side receiving the command being referred to as the target. The target then responds by sending a control signal (referred to as a "response") indicating command execution results to the controller as necessary. A series of exchanges starting with a command transmission and ending with a response are referred to as a command transaction. The controller is then capable of requesting specific operations of the target and interrogating the current conditions of the target with command transactions. Any apparatus within the system is capable of starting and ending command transactions with any apparatus being capable of becoming a controller or a target.
FIG. 2 is a block view showing the configuration of a portion for carrying out transmission and reception of control signals within each apparatus. A physical layer control block (PHY) 31, a link layer control block (LINK) 32 and a CPU 33 are provided within an apparatus 30. The physical layer control block carries out arbitration etc. for the initialization of the bus and the priority of use of the bus, as well as carrying out communication with the link layer control block 32 for the various control signals and transmission and reception of these signals to and from the cable of the 1394 serial bus 34. The link layer control block 32 carries out packet making/detection and error correction processing. The CPU 33 controls the physical layer control block 31 and the link layer control block 32 and carries out processing for the application layers for making the commands and responses etc. When commands and responses are being made, the CPU 33 writes data to prescribed addresses of a register provided within the link layer control block 32. Further, commands and responses transmitted by other apparatus are read by the CPU 33 after being written to a prescribed address of the register.
With the above system it was necessary for the clocks within the apparatus to be set to the same time for each apparatus regardless of whether or not the apparatus was connected to the 1394 bus and there has been no means for accurately setting the clock of each apparatus connected to the same 1394 bus to the same time.
Further, in the above described system, in order for a certain apparatus to set all of the other apparatus at standby condition, it is necessary to transmit command one by one from a certain apparatus to other apparatus.
As the present invention sets out to resolve the above problems, it is an object of the present invention to achieve time setting or state settings at each apparatus with just one communication.